Method of magnetizing the shaft of a linear stepper motor

ABSTRACT

In a preferred embodiment, a method of providing axially alternating N and S poles in a portion of an axially extending, cylindrical, smooth shaft for a linear stepper motor, including: providing a magnetizing fixture including: a hollow cylindrical mandrel formed from a non-magnetic material; a conductive wire disposed in parallel, circumferential channels defined in an outer surface of the mandrel; a potting compound surrounding the mandrel to secure the conductive wire in place; and a central bore defined axially and centrally through the mandrel and exposing or nearly exposing the conductive wire; and the central bore being sized to accept axially inserted therein the portion of the axially extending, cylindrical, smooth shaft; inserting the portion of the axially extending, cylindrical shaft in the central bore; and providing a direct current through the conductive wire, the conductive wire is placed in the parallel, circumferential channels such that direction of flow in the conductive wire of a direct current in adjacent ones of the parallel, circumferential channels is in opposite directions. A method of manufacturing the magnetizing fixture is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of co-pending U.S.application Ser. No. 10/794,148, filed Mar. 8, 2004, and titled FIXTUREFOR THE MAGNETIZATION OF THE SHAFT OF A LINEAR STEPPER MOTOR AND METHODSOF USING AND MANUFACTURING THE SAME, which is a divisional applicationof U.S. application Ser. No. 09/783,179, filed Feb. 12, 2001, and titledLINEAR STEPPER MOTOR AND FIXTURE FOR THE MAGNETIZATION OF THE SHAFTTHEREOF AND METHODS, now U.S. Pat. No. 6,756,705, issued Jun. 29, 2004,and titled LINEAR STEPPER MOTOR, which claims the benefit of the filingdates of U.S. Provisional Applications Nos. 60/181,449, filed February10, 2000, and titled LINEAR STEPPER MOTOR and 60/220,369, filed Jul. 24,2000, and titled METHOD AND FIXTURE FOR MANUFACTURE OF A LINEAR STEPPERMOTOR SHAFT.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to stepper motors generally and, moreparticularly, but not by way of limitation, to a novel method ofmagnetizing the shaft of a linear stepper motor.

2. Background Art

Some linear stepper motors convert rotary motion to linear motion bymechanical means such as through the use of a threaded nut and leadscrew. Conventional linear motors that directly transfer electromagneticenergy in the stator poles to linear movement of a shaft typicallyemploy toothed structures or have relatively complicated slide/statorarrangements. In either case, the manufacture of such motors isrelatively expensive and the motors typically have high parts counts.

A problem resides in producing a linear motor with a smooth shaft withalternating N and S poles. One technique is to glue together cylindricalsegments of N and S magnets. That technique, however, is time consumingand results in a somewhat weak structure. Another technique is to roll acylinder of ferromagnetic material over a flat plate orthogonal to aseries of alternating N and S magnetic strips. This technique issomewhat clumsy and suffers from the fact that the resulting magnetizedshaft is of fairly weak magnetic strength.

Some conventional motors are described in the following patentdocuments:

U.S. Pat. No. 3,867,676, issued Feb. 18, 1975, to Chai et al., andtitled VARIABLE RELUCTANCE LINEAR STEPPER MOTOR, describes such a motorthat has toothed structures on the coils and on the linear member. Thenovelty of the patent appears to reside in the arrangement of the coilsand the manner in which they are energized.

U.S. Pat. No. 4,198,582, issued Apr. 15, 1980, to Matias et al., andtitled HIGH PERFORMANCE STEPPER MOTOR, describes, in part, a variablereluctance linear stepper motor in which both the stator and the sliderhave nonmagnetic materials arranged therein such that flux leakage isreduced.

U.S. Pat. No. 4,286,180, issued Aug. 25, 1981, to Langley, and titledVARIABLE RELUCTANCE STEPPER MOTOR, describes, in part, such a motorhaving helically toothed stator and slide structures, the respectivewidths of the teeth having a predetermined relationship.

U.S. Pat. No. 4,408,138, issued Oct. 4, 1983, to Okamoto, and titledLINEAR STEPPER MOTOR, describes a linear stepper motor having toothedstructures on the stator and on the slider. Coil-wound salient poles areprovided on the slider. The novelty of the patent appears to reside inthe arrangement of rollers and rails disposed between the stator and theslider.

U.S. Pat. No. 4,607,197, issued Aug. 19, 1986, to Conrad, and titledLINEAR AND ROTARY ACTUATOR, describes a variable reluctancelinear/rotary motor in which the armature has axial rows of teethradially spaced around the surface thereof. Selective energization ofstator windings provides linear, rotary, or both linear and rotarymotion of the armature.

U.S. Pat. No. 4,622,609, issued Nov. 11, 1986, to Barton, and titledREAD/WRITE HEAD POSITIONING APPARATUS, describes a variable reluctancepositioning device having toothed structures on facing surfaces of thestator and the armature and with coils placed on the armature.

U.S. Pat. No. 4,695,777, issued Sep. 22, 1987, to Asano, and titled VRTYPE LINEAR STEPPER MOTOR, describes such a motor having toothedstructures on the stator and on the slider, the toothed structures onthe stator being on coil-wound salient poles. The toothed structuresbear a predetermined relationship therebetween.

U.S. Pat. No. 4,712,027, issued Dec. 8, 1987, to Karidis, and titledRADIAL POLE LINEAR RELUCTANCE MOTOR, describes such a motor having asmooth double-helix stator shaft and a smooth laminated armature ofalternate radial pole laminations and spacer laminations. Thisarrangement permits a balanced flux path and uses the stator andarmature surfaces as slider bearing surfaces.

U.S. Pat. No. 4,810,914, issued Mar. 7, 1989, to Karidis et al., andtitled LINEAR ACTUATOR WITH MULTIPLE CLOSED LOOP FLUX PATHS ESSENTIALLYORTHOGONAL TO ITS AXIS, describes a variable reluctance actuator similarin pertinent respects to that described in the '027 patent above.

U.S. Pat. No. 6,016,021, issued Jan. 18, 2000, to Hinds, and titledLINEAR STEPPER MOTOR, describes a variable reluctance stepper motorsimilar in pertinent respects to the motor described in the '609 patentabove. The novelty of the patent appears to reside in the method offorming the teeth.

Accordingly, it is a principal object of the present invention toprovide a method of magnetizing a permanent magnet shaft for a linearstepper motor that has a smooth, external peripheral surface.

It is a further object of the invention to provide such a method that isquick and economical.

Other objects of the present invention, as well as particular features,elements, and advantages thereof, will be elucidated in, or be apparentfrom, the following description and the accompanying drawing figures.

SUMMARY OF THE INVENTION

The present invention achieves the above objects, among others, byproviding, in a preferred embodiment, a method of providing axiallyalternating N and S poles in a portion of an axially extending,cylindrical, smooth shaft for a linear stepper motor, comprising:providing a magnetizing fixture comprising: a hollow cylindrical mandrelformed from a non-magnetic material; a conductive wire disposed inparallel, circumferential channels defined in an outer surface of saidmandrel; a potting compound surrounding said mandrel to secure saidconductive wire in place; and a central bore defined axially andcentrally through said mandrel and exposing or nearly exposing saidconductive wire; and said central bore being sized to accept axiallyinserted therein said portion of said axially extending, cylindrical,smooth shaft; inserting said portion of said axially extending,cylindrical shaft in said central bore; and providing a direct currentthrough said conductive wire said conductive wire is placed in saidparallel, circumferential channels such that direction of flow in saidconductive wire of a direct current in adjacent ones of said parallel,circumferential channels is in opposite directions. A method ofmanufacturing said magnetizing fixture is also provided.

BRIEF DESCRIPTION OF THE DRAWING

Understanding of the present invention and the various aspects thereofwill be facilitated by reference to the accompanying drawing figures,provided for purposes of illustration only and not intended to definethe scope of the invention, on which:

FIG. 1 is a fragmentary, side elevational view, partially incross-section, of a linear stepper motor constructed according to thepresent invention.

FIG. 2 is a rear elevational view of the motor of FIG. 1.

FIG. 3 is an isometric view of a grooved mandrel for use in fabricatinga fixture for magnetizing the shaft of the motor of FIG. 1.

FIG. 4 is an isometric view of the mandrel of FIG. 3 with a conductivewire inserted in the grooves of the mandrel.

FIG. 5 is a schematic, isometric view of the conductive wire showing thepath of a direct current flowing therein.

FIG. 6 is an isometric view, partially cut-away, showing the mandrel ofFIG. 3 inserted in a potting fixture.

FIG. 7 is an isometric view showing a magnetizing fixture according tothe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference should now be made to the drawing figures on which similar oridentical elements are given consistent identifying numerals throughoutthe various figures thereof, and on which parenthetical references tofigure numbers direct the reader to the view(s) on which the element(s)being described is (are) best seen, although the element(s) may be seenon other figures also.

FIG. 1 illustrates a linear stepper motor, constructed according to thepresent invention, and generally indicated by the reference numeral 20.Motor 20 includes a shaft, or slider, 30 having a smooth outerperipheral surface (at least the portion thereof illustrated) andinserted in a stator structure, generally indicated by the referencenumeral 32, for axial back and forth motion of the shaft with respect tothe stator structure.

Shaft 30 includes a plurality of alternating N and S nonsalient poles,as at 34 and 36, respectively, formed around the periphery thereof,which poles may be formed as described below. Shaft 30 is preferably ahollow cylinder of ceramic or rare earth magnetic material, although theshaft may be solid or may have a core of ferromagnetic or other materialwith a hollow cylinder of the magnetic material disposed around thecore. Shaft 30 can be economically constructed, for example, byconventional extrusion techniques that can produce a shaft of any givenlength or the shaft can be cut to a suitable length from extruded stock.At least the portion of shaft 30 containing the N and S poles isnon-segmented and is constructed of a single piece of material.

Stator structure 32 includes first and second, cylindrical, coils 40 and42, respectively, encircling shaft 30, and conventionally wound on firstand second annular bobbins 44 and 46. Bobbins 44 and 46 are formed of anelectrically insulating material such as Delrin®. First and secondbobbins 44 and 46 are spaced apart by a first spacer 50 and the secondbobbin may be spaced apart from an end plate 52 of motor 20 by a secondspacer 54. First and second spacers 50 and 54 may also provide bearingsurfaces for shaft 30, in which case the first and second spacers arepreferably of a material having a high degree of lubricity such asDelrin®.

First bobbin 44 spaces apart annular pole plates 60 and 62, while secondbobbin 46 spaces apart annular pole plates 64 and 66. A steel band 68surrounds and is in good electrical contact with annular pole plates 60,62, 64, and 66, thus completing the circular electromagnetic circuit.Annular pole plates 60, 62, 64, and 66 have nonsalient poles.

It will be understood that, by suitable energization of first and secondcoil-wound bobbins in a conventional manner, shaft 30 may be made toincrementally “step” to the left or right on FIG. 1. It will be furtherunderstood that one or both ends of shaft 30 may be attached to, or bearagainst, one or more elements of another device (not shown).

While motor 20 is shown as having one set of two-phase stator sections,that is, the motor has two coils, it will be understood that otherarrangements are possible as well. For example, two or more sets oftwo-phase stator sections may be provided for greater power, theadditional sets of stator sections being added serially in a modularmanner.

FIG. 2 illustrates some of the elements of motor 20 (FIG. 1) andillustrates conductors 70 that are used to energize stator structure 32and mounting holes 72 defined in end plate 52.

Thus arranged, motor 20 as shown (FIG. 1) in its minimum configurationis constructed of only 11 individual elements that may be held togetherprincipally with a suitable adhesive or other conventional means may beprovided to secure together the elements of motor 20.

FIG. 3 illustrates a mandrel 100 that can be used in constructing afixture for use in magnetizing shaft 30 (FIG. 1). Here a cylindricalmandrel 100 has a plurality of parallel, cylindrical grooves, as at 110,cut in the outer periphery thereof, the groove having a widthapproximating the diameter of a wire conductor to be used in magnetizingshaft 30. Mandrel 100 is constructed of a non-magnetic,non-electrically-conducting material, with the spacing of grooves 110being determined by the final magnetic widths of poles 34 and 36 onshaft 30.

FIG. 4 illustrates a conductive wire 150 serially disposed in grooves110 in mandrel 100.

FIG. 5 illustrates the current path in conductive wire 150, each nearlycomplete circle shown on FIG. 5 representing a turn of conductive wire150 in one of grooves 110. It will be noticed that the current flowrepresented by the arrows in conductive wire 150 in adjacent turns ofthe conductive wire are in opposite directions.

FIG. 6 illustrates mandrel 100, with conductive wire 150 placed ingrooves 110, disposed in a cylindrical, hollow potting fixture 200. Inthis step, a suitable potting compound, such as an epoxy material, ispoured into an annulus 210 defined between the outer surface of mandrel100 and the inner surface of potting fixture 200. After hardening, thepotting compound holds conductive wire 150 in place in grooves 110.

FIG. 7 illustrates a finished magnetizing fixture, generally indicatedby the reference numeral 300. Fixture 300 comprises mandrel 100 with anouter coating of potting compound 310 and ends of conductive wire 150extending therefrom. A central axial bore 320 has been created, orenlarged, through mandrel 100 to bring conductive wire 150 near to theinner surface of the mandrel or even to be partially exposed, as shownon FIG. 7, if desired.

Shaft 30 of motor 20 (FIG. 1) can now be inserted into fixture 300 and ahigh level of direct current passed through conductive wire 150 tomagnetize alternating N and S poles 34 and 36 along a selected lengththereof. Such an arrangement provides an economical and rapid method ofmagnetizing shaft 30 and nearly any strength of magnetization can beprovided, depending on the magnet material, since only one quick burstof direct current is necessary and that can be in a wide range ofvoltages.

Motor 20 (FIG. 1) has a number of important features. For example, motor20 is of a brushless, magnetically coupled, bi-directional, non-arcingdesign, having long operational life, with permanently magnetized outputshaft 30. Motor 20 runs on conventional stepper motor drives and can bemicrostepped for increased resolution and accuracy. Shaft 30 is the onlymoving part and it can be rotated 360° continuously or intermittently ineither direction, at any time and at any linear position, including whenmotor 20 is not energized. There is no conversion of rotary motion tolinear motion with the concomitant efficiency losses. There are no leadscrews, ball screws, or ball bearings to wear out and no lubrication isrequired. Motor 20 can operate in any orientation and is back-driveable(especially at low or zero power input), that is, shaft 30 can be movedby overcoming the magnetic force between the shaft and annular poleplates 64 and 66. Performance of motor 20 can be increased with shorterduty cycles and can be easily constructed for vacuum environments, thatis, it can be constructed of materials that do not out gas in a vacuum,the lack of lubrication contributing to this feature. Shaft 30 whenhollow allows the pass-through of electrical, optical, and/or fluidlines, and/or the like.

In the embodiments of the present invention described above, it will berecognized that individual elements and/or features thereof are notnecessarily limited to a particular embodiment but, where applicable,are interchangeable and can be used in any selected embodiment eventhough such may not be specifically shown.

Terms such as “upper”, “lower”, “inner”, “outer”, “inwardly”,“outwardly”, “vertical”, “horizontal”, and the like, when used herein,refer to the positions of the respective elements shown on theaccompanying drawing figures and the present invention is notnecessarily limited to such positions.

It will thus be seen that the objects set forth above, among thoseelucidated in, or made apparent from, the preceding description, areefficiently attained and, since certain changes may be made in the aboveconstruction without departing from the scope of the invention, it isintended that all matter contained in the above description or shown onthe accompanying drawing figures shall be interpreted as illustrativeonly and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

1. A method of providing axially alternating N and S poles in a portionof an axially extending, cylindrical, smooth shaft for a linear steppermotor, comprising: (a) providing a magnetizing fixture comprising: ahollow cylindrical mandrel formed from a nonmagnetic material; a singleconductive wire serially disposed in parallel, circumferential channelsdefined in an outer surface of said mandrel; a potting compoundsurrounding said mandrel to secure said single conductive wire in place;and a central bore defined axially and centrally through said mandreland exposing or nearly exposing said conductive wire; and said centralbore being sized to accept axially inserted therein said portion of saidaxially extending, cylindrical, smooth shaft; (b) inserting said portionof said axially extending, cylindrical shaft in said central bore; and(c) providing a direct current through said single conductive wire tomagnetize alternating N and S poles along the portion of the axiallyextending cylindrical shaft.
 2. The method according to claim 1, whereinsaid conductive wire is placed in said parallel, circumferentialchannels such that direction of flow in said single conductive wire of adirect current in adjacent ones of said parallel, circumferentialchannels is in opposite directions.
 3. A method of manufacturing amagnetizing fixture for magnetizing axially alternating N and S polesdefined circumferentially in a portion of an outer periphery of anaxially extending, cylindrical, smooth shaft, said method comprising:(a) providing a plurality of parallel circumferential channels definedin an outer surface of a cylindrical mandrel formed from a non-magneticmaterial; (b) placing a single conductive wire serially in saidparallel, circumferential channels; (c) providing a potting compoundsurrounding said mandrel to secure said single conductive wire in place;(d) forming a central bore defined axially and centrally through saidmandrel and exposing or nearly exposing said single conductive wire; and(e) said central bore being sized to accept axially inserted thereinsaid portion of said axially extending, cylindrical smooth shaft.
 4. Themeted according to claim 3, wherein said conductive wire is placed insaid parallel, circumferential channels such that direction of flow insaid single conductive wire of a direct current in adjacent ones of saidparallel, circumferential channels is in opposite directions.
 5. Themethod according to claim 2, wherein the single conductive wire isinserted in a first of said parallel, circumferential chambers in afirst turn in a first direction of nearly 360 degrees and exits themandrel to form an external loop and then re-enters the mandrel in asubsequent parallel, circumferential chamber to form a next turn in anopposite direction of nearly 360 degrees, and then repeats this patternfor the remainder of the parallel circumferential chambers on themandrel, whereby alternating magnetic poles are produced along thelength of the shaft.
 6. The method according to claim 4, wherein thesingle conductive wire is inserted in a first of said parallel,circumferential chambers in a first turn in a first direction of nearly360 degrees and exits the mandrel to form an external loop and thenre-enters the mandrel in a subsequent parallel, circumferential chamberto form a next turn in an opposite direction of nearly 360 degrees, andthen repeats this pattern for the remainder of the parallelcircumferential chambers on the mandrel, whereby alternating magneticpoles are produced along the length of the shaft.